For example, when an emulsion polymerized copolymer of tetrafluoroethylene and hexafluoropropylene is melt processed to obtain a final product, bubbles or voids may form due to volatile materials in the final product. The volatile materials are generated from polymer chain terminals and polymer backbones which are unstable against heat and/or shear force. If such unstable sites are stabilized during melt processing, the volatile materials still remain in the polymer processed, and thus the bubbles and voids may form when the polymer is processed to the final product.
JP-A-56-44883 discloses a method for reducing the content of volatile materials in a tetrafluoroethylene-hexafluoropropylene copolymer to 70% or less of the initial content of the volatile materials by heating the copolymer which is statically placed on a pan or a mesh in an open condition, for example, in an electric furnace. However, the treatment in the open condition can hardly avoid the migration of foreign materials from outside to the heating process. Although the content of the volatile materials can be reduced by static heating, when the filling depth of the copolymer increases, the treating time is prolonged with the influence of diffusion of the volatile materials, and also the treatment inside the layer of the copolymer filled becomes less uniform. To decrease the filling depth of the copolymer, a dish or a mesh having a large area is necessary. As a result, a large apparatus is required.